Satellite-derived constraints on the effect of drought stress on biogenic isoprene emissions in the southeastern US

Abstract

Abstract. While substantial progress has been made to improve our understanding of biogenic isoprene emissions under unstressed conditions, large uncertainties remain with respect to isoprene emissions under stressed conditions. Here, we use the US Drought Monitor (USDM) as a weekly drought severity index and tropospheric columns of formaldehyde (HCHO), the key product of isoprene oxidation, retrieved from the Ozone Monitoring Instrument (OMI) to derive top-down constraints on the response of summertime isoprene emissions to drought stress in the southeastern United States (SE US), a region of high isoprene emissions that is also prone to drought. OMI HCHO column density is found to be 6.7 % (mild drought) to 23.3 % (severe drought) higher than that under non-drought conditions. A global chemical transport model, GEOS-Chem, with version 2.1 of the Model of Emissions of Gases and Aerosols from Nature (MEGAN2.1) emission algorithm can simulate this direction of change, but the simulated increases at the corresponding drought levels are 1.1–1.5 times that of OMI HCHO, suggesting the need for a drought-stress algorithm in the model. By minimizing the model–OMI differences in HCHO to temperature sensitivity under different drought levels, we derived a top-down drought stress factor (γd_OMI) in GEOS-Chem that parameterizes using water stress and temperature. The algorithm led to an 8.6 % (mild drought) to 20.7 % (severe drought) reduction in isoprene emissions in the SE US relative to the simulation without it. With γd_OMI the model predicts a nonlinear increasing trend in isoprene emissions with drought severity that is consistent with OMI HCHO and a single site's isoprene flux measurements. Compared with a previous drought stress algorithm derived from the latter, the satellite-based drought stress factor performs better with respect to capturing the regional-scale drought–isoprene responses, as indicated by the near-zero mean bias between OMI and simulated HCHO columns under different drought conditions. The drought stress algorithm also reduces the model's high bias in organic aerosol (OA) simulations by 6.60 % (mild drought) to 11.71 % (severe drought) over the SE US compared to the no-stress simulation. The simulated ozone response to the drought stress factor displays a spatial disparity due to the isoprene-suppressing effect on oxidants, with an <1 ppb increase in O3 in high-isoprene regions and a 1–3 ppbv decrease in O3 in low-isoprene regions. This study demonstrates the unique value of exploiting long-term satellite observations to develop empirical stress algorithms on biogenic emissions where in situ flux measurements are limited.

Observed and CMIP6 model simulated organic aerosol response to drought in the contiguous United States during summertime

“…This indicates more OA formations per unit increase in sulfate as drought deteriorates. The higher sensitivities of OA to sulfate under droughts can be explained by increasingly higher isoprene concentrations as shown in our previous studies in the SEUS (Wang et al, 2022b;Li et al, 2022), resulting in more IEPOX in the atmosphere to be further converted to particle phase catalyzed by sulfate. The intercept of the linear regression can be interpreted as other OA components that are not associated with sulfatecatalyzed IEPOX SOA, such as POA and anthropogenic SOA (Malm et al, 2017).…”

Disentangling temperature and water stress contributions to trends in isoprene emissions using satellite observations of formaldehyde, 2005–2016

“…In addition, these findings suggest that the effect of water stress on isoprene emissions differs from one region to another. Using OMI HCHO columns over the South-Eastern United States, Wang et al (2022) found that HCHO is 6% higher during mild drought and 23% during severe droughts. These results support future developments in biogenic emission models to implement algorithms able to reproduce both regimes (mild/short water stress, increasing emissions; severe/long water stress, decreasing emissions) and different vegetation/region sensitivity to water stress.…”

Enhancing Long-Term Trend Simulation of Global Tropospheric OH and Its Drivers from 2005–2019: A Synergistic Integration of Model Simulations and Satellite Observations

“…We observe HCHO columns going up in the northwestern US and over oil sands in Canada, possibly due to increased evergreen needleleaf forest and an increase in crude oil production (Zhu et al, 2017), respectively. The downward trends over the southeast US could be due to a decrease in drought events (Figure S5), which significantly affect isoprene emissions and the oxidation of VOCs (Duncan et al 2009;Naimark et al, 2021;Wang et al, 2022). Alternatively, this downward trend could be partially due to the dampened HCHO production from VOC oxidation due to reduced NOx emissions (Marais et al, 2014;Wolfe et al, 2016;Souri et al, 2020c).…”